Aluminum metal is conveniently produced in an electrolytic cell by passing a current through a bath of molten cryolite containing dissolved alumina. The cell is typically a large tank or cell lined with carbon, with the lining serving as part of the cathode system. Large carbon blocks extend into the top of the bath to function as the anode. Molten aluminum metal collects as a metal pad at the bottom of the cell. The cell typically has a bottom lining made up of cemented carbon blocks in which horizontally disposed steel cathode bars extend from the opposite sides of the cell. These cathode bars are connected to the cathode collector blocks by means of cast iron, and the blocks are anchored in a side lining.
Most aluminum reduction cells in commercial use employ prefabricated carbon blocks as the cell lining and as the cathodic working surface. These blocks provide high operating strength, high density, and lower electrical resistivity than that of continuous rammed paste type of linings. The blocks are formed into a liquid-tight container surfaced by filling the joints between the blocks with a ramming paste. The efficiency of sealing of the ramming paste is an important factor in determining the life and energy efficiency of a reduction cell, which depends to a great degree on the extent and rate of electrolytic penetration into the cell bottom.
The anodes are typically made from crushed petroleum coke and liquid pitch, which is formed into rectangular blocks and baked. These anode blocks are attached to rods and suspended into the electrolytic cell where they are slowly consumed in the aluminum smelting process.
They must be replaced after several weeks and the remaining portions attached to the rods that are removed are known as “anode butts”. These butts are typically recycled.
The cathode blocks are traditionally made from an anthracite aggregate mixed with a pitch binder. Graphite components can be substituted to increase electrical conductivity. As mentioned above, ramming paste is used to fill the spaces and form seams between individual cathode blocks, as well as to connect the side walls with the cathode blocks. A typical hot ramming paste consists of an anthracite filler and a coal tar binder. A cold (room temperature) paste binder usually includes a diluent to lower its softening point.
For making the joints with ramming paste, the paste is added to the joint spaces and compacted, e.g. with pneumatic hammers. The joints are formed in several layers to a final top surface flush with the top of the cathode blocks. Side walls, including monolithic slopes, can be formed in a similar manner by compacting the paste.
Premature cell failure can occur because of infiltration of metal into the joints, and the problem can be worsened when graphitised cathode blocks are used, which have a very low sodium swelling index. Because of this problem, there is a need for a high swelling, cold ramming paste which can serve to tighten the peripheral joint (big joint) and joints between the cathode blocks.
Another problem that can occur with these electrolytic cells is, because of occasional excessive metal motion due to magneto-hydrodynamic effects (MHE), the cathode lining and, particularly the amount of monolithic side slope and the joints between the cathode blocks, can be subjected to rapid erosion and failure.
Reamey et al. U.S. Pat. No. 3,871,986 describes a ramming cement for an aluminum reduction cell which is described as not shrinking when subsequently baked. That required a special pitch binder comprising a petroleum pitch having a cube-in-water softening point between about 40° and about 85° C. and having a content of material insoluble in quinoline not greater than about 1%.
U.S. Pat. No. 5,961,811 (Keller) describes another form of ramming paste for an aluminum reduction cell made of carbon and a reactive compound, such as a carbide, fluoride, phosphate or oxide compound, capable of reacting with titanium or zirconium to produce titanium or zirconium diboride during operation of the cell to produce aluminum. The titanium or zirconium diboride is produced in an amount sufficient to improve molten aluminum wetting properties of the carbonaceous material.
In Mirtchi, U.S. Pat. No. 6,258,224, the problem had to do with the erosion/corrosion of bottom blocks of an electrolytic cell for producing aluminum because of the movement of cell contents caused by MHE. That patent provided a multi-layer cathode structure including a carbonaceous cathode substrate and at least one layer of a metal boride, e.g. TiB2, containing composite refractory material over the substrate. The inner face between the substrate and the TiB2 composite material was first roughened (raked) to overcome thermal expansion differences between the two materials.
Another attempt at improving ramming paste for aluminum reduction cells is described in de Nora et al. U.S. Pat. No. 5,676,807. The main concern in de Nora was the polluting affect of pitch binders and, according to that invention, the ramming paste was produced using a colloidal binding material, e.g. alumina in colloidal form.
It is an object of the present invention to provide an improved ramming paste that has a high swelling index and can be used under cold, e.g. 30° C., conditions.
It is a further object of this invention to provide an improved ramming paste having good erosion resistance and which is wettable by aluminum.